As the semiconductor industry introduces new generations of integrated circuits (IC's) having higher performance and greater functionality, the density of the elements that form those IC's is increased, while the dimensions, sizes, and spacing between the individual components or elements are reduced. While in the past such reductions were limited only by the ability to define the structures photo-lithographically, device geometries having smaller dimensions created new limiting factors. For example, for any two adjacent conductive paths, as the distance between the conductors decreases, the resulting capacitance (a function of the dielectric constant (k) of the insulating material divided by the distance between conductive paths) increases. This increased capacitance results in increased capacitive coupling between the conductors, increased power consumption, and increased resistive-capacitive (RC) delay. Therefore, the continual improvement in the performance and functionality of semiconductor IC's depend upon developing dielectric materials having lower dielectric constants (k) than that of the most commonly used silicon oxide, thus resulting in reduced capacitance.
Low-k dielectric materials typically require a curing process subsequent to the deposition in order to increase their porosities, lower their k values, and improve their mechanical strengths. Typical curing methods include thermal curing, plasma curing, and ultra violet (UV) curing. Among the three methods, plasma and UV curing are performed at substantially shorter times or at lower temperatures, eliminating the need for prior furnace curing, and hence reducing the total thermal budget.
Porous films are mechanically weak by nature. Weak films may fail in the chemical mechanical polishing (CMP) processes employed to planarize the wafer surface during chip manufacturing. Further, the weak low-k dielectric materials cause difficulties in the packaging processes. For example, when wafers are sawed, the low-k dielectric materials in scribe lines may peel off. In addition, in wire bonding processes, the force applied for detaching wires also causes the low-k dielectric materials underlying the bond pads to peel off. Both situations may cause circuit failure. Accordingly, performing an efficient curing to maximize the mechanical strength of low-k dielectric materials becomes very important.
FIG. 1 schematically illustrates a cross-sectional view of an integrated circuit structure in a curing stage, in which extreme low-k (ELK) layer 2, etch stop layer 4, and ELK layer 6 are shown. After ELK layer 6 is deposited, it is cured using a UV curing with a UV light, as is symbolized by arrow 8. To efficiently cure ELK 6, it is desirable that the UV energy is absorbed by ELK 6 as much as possible. However, a significant portion of UV energy penetrates through ELK 6, and goes into etch stop layer 4 and the underlying integrated circuits, including ELK 2. The efficiency of the UV curing is thus low. A new method for forming and curing low-k dielectric materials is thus needed.